Physiological and Mechanistic Studies of Phototrophic Fe(II) Oxidation in Purple Non-sulfur Bacteria

Abstract

Phototrophic Fe(II)-oxidizing bacteria use electrons from ferrous iron [Fe(II)] and energy from light to drive reductive CO₂ fixation. This metabolism is thought to be ancient in origin, and plays an important role in environmental iron cycling. It has been implicated in the deposition of Banded Iron Formations, a class of ancient sedimentary iron deposits. Consistent with this hypothesis, we discovered that hydrogen gas, a thermodynamically favorable electron donor to Fe(II), in an Archean atmosphere would not have inhibited phototrophic Fe(II) oxidation. To understand this physiology and the connection to BIF formation at the molecular level, the mechanisms of phototrophic Fe(II) oxidation were examined in two purple non-sulfur bacteria, Rhodopseudomonas palustris TIE-1 and Rhodobacter sp. SW2. Important advances were made in elucidating genes critical to phototrophic Fe(II) oxidation. In R. palustris TIE-1, the first genetically tractable phototrophic Fe(II) oxidizer isolated, transposon mutagenesis identified a putative integral membrane protein and a potential cobalamin (vitamin B₁₂) biosynthesis protein involved in Fe(II) oxidation. Increased expression of a putative decaheme c-type cytochrome, encoded by pioA, was observed when cells were grown under Fe(II)-oxidizing conditions. Two genes located immediately downstream of pioA in the same operon, pioB and pioC, encode a putative outer membrane beta-barrel protein and a putative high potential iron-sulfur protein, respectively. Deletion studies demonstrated that all three genes are involved in phototrophic Fe(II) oxidation. In Rhodobacter sp. SW2, a three-gene operon, foxEYZ, was found to be involved in phototrophic Fe(II) oxidation through heterologous expression in a close relative, Rhodobacter capsulatus SB1003. The first gene, foxE, encodes a novel c-type cytochrome located in the periplasm. Expression of foxE alone confers light-dependent Fe(II) oxidation activity to SB1003, but maximal activity is achieved when foxE is co-expressed with foxY and foxZ. FoxY appears to contain the redox cofactor pyrroloquinoline quinone and FoxZ a cytoplasmic membrane transporter. Recombinant PioC was overexpressed and partially purified from Escherichia coli. This research presents a detailed study of the physiology and genetics of phototrophic Fe(II) oxidation in two purple non-sulfur bacteria, and provides our first insight into the molecular mechanisms of this metabolism.